Nitride semiconductor structure and semiconductor light emitting device including the same

ABSTRACT

A nitride semiconductor structure and a semiconductor light emitting device including the same are revealed. The nitride semiconductor structure includes a multiple quantum well structure formed by a plurality of well layers and barrier layers stacked alternately. One well layer is disposed between every two barrier layers. The barrier layer is made of Al x In y Ga 1-x-y N (0&lt;x&lt;1, 0&lt;y&lt;1, 0&lt;x+y&lt;1) while the well layer is made of In z Ga 1-z N (0&lt;z&lt;1). Thereby quaternary composition is adjusted for lattice match between the barrier layers and the well layers. Thus crystal defect caused by lattice mismatch is improved.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of and claims the priority benefit ofU.S. application Ser. No. 13/963,109, filed on Aug. 9, 2013, nowallowed, which claims the priority benefit of Taiwan application serialno. 101143101, filed on Nov. 19, 2012. The entirety of each of theabove-mentioned patent applications is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor structure and asemiconductor light emitting device including the same, especially to anitride semiconductor structure that has a multiple quantum wellstructure formed by quaternary AlGaInN barrier layers and ternary InGaNwell layers for reducing stress coming from lattice mismatch. Thethickness of the well layer is ranging from 3.5 nm to 7 nm. At the sametime, a better carrier confinement is provided and the internal quantumefficiency is improved. Thus the semiconductor light emitting device hasa better light emitting efficiency.

2. Description of Related Art

Generally, a nitride light emitting diode is produced by forming abuffer layer on a substrate first. Then a n-type semiconductor layer, alight emitting layer and a p-type semiconductor layer are formed on thebuffer layer in turn by epitaxial growth. Next use photolithography andetching processes to remove a part of the p-type semiconductor layer anda part of the light emitting layer until a part of the n-typesemiconductor layer is exposed. Later a n-type electrode and a p-typeelectrode are respectively formed on the exposed n-type semiconductorlayer and the p-type semiconductor layer. Thus, a light emitting diodedevice is produced. The light emitting layer has a multiple quantum well(MQW) structure formed by a plurality of well layers and barrier layersdisposed alternately. The band gap of the well layer is lower than thatof the barrier layer so that electrons and holes are confined by eachwell layer of the MQW structure. Thus electrons and holes arerespectively injected from the n-type semiconductor layer and the p-typesemiconductor layer to be combined with each other in the well layersand photons are emitted.

In the MQW structure, there are about 1-30 layers of well layers orbarrier layers. The barrier layer is usually made of GaN while the welllayer is made of InGaN. However, there is about 10˜15% lattice mismatchbetween GaN and InGaN that causes a large stress in the lattice. Thus apiezoelectric field is induced in the MQW structure by the stress.Moreover, during growth of InGaN, the higher indium composition, thelarger the piezoelectric field generated. The piezoelectric field has agreater impact on the crystal structure. The stress accumulated isgetting larger along with the increasing thickness during growth ofInGaN. After the crystal structure being grown over a criticalthickness, larger defects (such as V-pits) are present due to thestress, so that the thickness of the well layer has a certain limit,generally about 3 nm.

Moreover, in the MQW structure, band gap is tilted or twisted due toeffects of a strong polarization field. Thus electrons and holes areseparated and confined on opposite sides of the well layer, which leadsto decrease the overlapping of the wave function of the electron holepairs and further to reduce both radiative recombination rate andinternal quantum efficiency of electron hole pairs.

SUMMARY OF THE INVENTION

A nitride semiconductor structure comprising a first type dopedsemiconductor layer; a light emitting layer, comprising a multiplequantum well (MQW) structure; an AlGaN based second type carrierblocking layer; and a second type doped semiconductor layer, wherein theAlGaN based second type carrier blocking layer is disposed between thesecond type doped semiconductor layer and the light emitting layer, andthe light emitting layer is disposed between the AlGaN based second typecarrier blocking layer and the first type doped semiconductor layer, andthe MQW structure comprises a plurality of AlInGaN based barrier layersand a plurality of InGaN based well layers stacked alternately.

A nitride semiconductor structure comprising: a first type dopedsemiconductor layer; a light emitting layer, comprising a multiplequantum well (MQW) structure; a InGaN based hole supply layer; and asecond type doped semiconductor layer, wherein the light emitting layeris disposed between the first type doped semiconductor layer and theInGaN based hole supply layer, and the InGaN based hole supply layer isdisposed between the light emitting layer and the second type dopedsemiconductor layer, and the MQW structure comprises a plurality ofAlInGaN based barrier layers and a plurality of InGaN based well layersstacked alternately, and the band gap of the hole supply layer is largerthan that of the InGaN based well layers.

A nitride semiconductor structure comprising: a first type dopedsemiconductor layer; a AlGaN based first type carrier blocking layer; alight emitting layer, comprising a multiple quantum well (MQW)structure; a AlGaN based second type carrier blocking layer; and asecond type doped semiconductor layer, wherein the light emitting layeris disposed between the first type doped semiconductor layer and thesecond type doped semiconductor layer, the AlGaN based first typecarrier blocking layer is disposed between the first type dopedsemiconductor layer and the light emitting layer, the AlGaN based secondtype carrier blocking layer is disposed between the second type dopedsemiconductor layer and the light emitting layer, and the MQW structurecomprises a plurality of AlInGaN based barrier layers and a plurality ofInGaN based well layers stacked alternately.

By the quaternary AlGaInN barrier layers and the ternary InGaN welllayers, the stress caused by lattice mismatch is improved and thepiezoelectric field in the MQW structure is further reduced effectively.Thus inhibition of the piezoelectric effect and improvement of internalquantum efficiency are achieved. Therefore the semiconductor lightemitting device gets a better light emitting efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and the technical means adopted by the present inventionto achieve the above and other objects can be best understood byreferring to the following detailed description of the preferredembodiments and the accompanying drawings, wherein

FIG. 1 is a schematic drawing showing a cross section of an embodimentof a nitride semiconductor structure according to the present invention;

FIG. 2 is a schematic drawing showing a cross section of an embodimentof a semiconductor light emitting device including a nitridesemiconductor structure according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following embodiments, when it is mentioned that a layer ofsomething or a structure is disposed over or under a substrate, anotherlayer of something, or another structure, that means the two structures,the layers of something, the layer of something and the substrate, orthe structure and the substrate can be directly or indirectly connected.The indirect connection means there is at least one intermediate layerdisposed therebetween.

Referring to FIG. 1, a first type doped semiconductor layer 3 and asecond type doped semiconductor layer 7 are disposed over a substrate 1.A light emitting layer 5 is disposed between the first type dopedsemiconductor layer 3 and the second type doped semiconductor layer 7.The light emitting layer 5 has a multiple quantum well (MQW) structure.The MQW structure includes a plurality of well layers 51 and barrierlayers 52 stacked alternately. One well layer 51 is disposed betweenevery two barrier layers 52. The barrier layer 52 is made of quaternaryAlxInyGa1-x-yN (0<x<1, 0<y<1, 0<x+y<1) and the well layer 51 is made ofmaterial In_(z)Ga_(1-z)N (0<z<1). The thickness of the well layer 51 isranging from 3.5 nm to 7 nm, preferably from 4 nm to 5 nm. The thicknessof the barrier layer 52 is ranging from 5 nm to 12 nm. The barrier layer52 is doped with a first type dopant (such as silicon or germanium) at aconcentration ranging from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³ so as to reducecarrier screening effect and increase carrier-confinement.

Moreover, a hole supply layer 8 is disposed between the light emittinglayer 5 and the second type doped semiconductor layer 7. The hole supplylayer 8 is made of In_(x)Ga_(1-x)N (0<x<1) and is doped with a secondtype dopant (such as magnesium or zinc) at a concentration larger than10¹⁸ cm⁻³. The hole supply layer 8 is also doped with a Group IV Aelement whose concentration is ranging from 10¹⁷ cm⁻³ to 10²⁰ cm⁻³. Theoptimal Group IV A element is carbon. The pentavalent nitrogen isreplaced by carbon, so that the hole supply layer 8 has higherconcentration of holes and more holes are provided to enter the lightemitting layer 5. Thus the electron-hole recombination is increased. Theband gap of the hole supply layer 8 is larger than that of the welllayer 51 of MQW structure, so that the holes are allowed to enter thewell layers and the electrons will not escape into the second type dopedsemiconductor layer 7.

Furthermore, a first type carrier blocking layer 4 made of materialAl_(x)Ga_(1-x)N (0<x<1) is disposed between the light emitting layer 5and the first type doped semiconductor layer 3 while a second typecarrier blocking layer 6 made of Al_(x)Ga_(1-x)N (0<x<1) is disposedbetween the hole supply layer 8 and the second type doped semiconductorlayer 7. Due to the property that the band gap of AlGaN containingaluminum is larger than that of the GaN, not only the range of band gapof the nitride semiconductor is increased, the carriers are confined inthe MQW structure. Thus the electron-hole recombination rate isincreased and the light emitting efficiency is improved.

In addition, a buffer layer 2 made of Al_(x)Ga_(1-x)N (0<x<1) isdisposed between the substrate 1 and the first type doped semiconductorlayer 3. The buffer layer 2 is for improving lattice mismatch caused bythe first type doped semiconductor layer 3 grown on the heterogeneoussubstrate 1. The materials for the buffer layer 2 can also be GaN,InGaN, SiC, ZnO, etc. The buffer layer is produced by a low-temperatureepitaxial growth at the temperature ranging from 400 degrees Celsius (°C.) to 900° C.

While in use, the material for the substrate 1 can be sapphire, silicon,SiC, ZnO or GaN, etc. The first type doped semiconductor layer 3 is madeof Si-doped or Ge-doped GaN-based materials while the second type dopedsemiconductor layer 7 is made of Mg-doped or Zn-doped GaN-basedmaterials. The first type doped semiconductor layer 3 and the secondtype doped semiconductor layer 7 are produced by the method such asmetalorganic chemical vapor deposition (MOCVD). As to the well layer 51and the barrier layer 52, they are produced by metal organic chemicalvapor deposition or molecular beam epitaxy (MBE) deposition of gasmixture of a lower alkyl group-indium and gallium compound. The barrierlayers 52 are deposited at the temperature ranging from 850° C. to 1000°C. while the well layers 51 are formed at the temperature ranging from500° C. to 950° C. The AlGaInN barrier layers 52 and the InGaN welllayers 51 of the MQW structure have the same element-indium so that thelattice constant of the barrier layers 52 and the lattice constant ofthe well layers 51 are similar. Thus not only crystal defects caused bylattice mismatch between conventional InGaN well layers and GaN barrierlayers can be improved, the stress caused by lattice constant mismatchbetween materials is also improved. The thickness of the well layer 51of the nitride semiconductor structure is ranging from 3.5 nm to 7 nm,preferably from 4 nm to 5 nm.

Moreover, the piezoelectric field in the MQW structure is effectivelyreduced because that the quaternary AlGaInN barrier layers 52 and InGaNwell layers 51 can improve the stress caused by lattice mismatch. Thusthe tilted and twisted energy band is improved in a certain degree.Therefore the piezoelectric effect is reduced effectively and theinternal quantum efficiency is increased.

The above nitride semiconductor structure is applied to semiconductorlight emitting devices. Referring to FIG. 2, a cross section of asemiconductor light emitting device including the nitride semiconductorstructure of an embodiment according to the present invention isrevealed. The semiconductor light emitting device includes at least: asubstrate 1, a first type doped semiconductor layer 3 disposed over thesubstrate 1 and made of Si-doped or Ge-doped GaN based materials, alight emitting layer 5 disposed over the first type doped semiconductorlayer 3 and having a multiple quantum well (MQW) structure, a secondtype doped semiconductor layer 7 disposed over the light emitting layer5 and made of Mg-doped or Zn-doped GaN based materials, a first typeelectrode 31 disposed on and in ohmic contact with the first type dopedsemiconductor layer 3, and a second type electrode 71 disposed on and inohmic contact with the second type doped semiconductor layer 7.

The MQW structure includes a plurality of well layers 51 and barrierlayers 52 stacked alternately. One well layer 51 is disposed betweenevery two barrier layers 52. The barrier layer 52 is made of materialAl_(x)In_(y)Ga_(1-x-y)N and x and y satisfy the conditions: 0<x<1,0<y<1, and 0<x+y<1 while the well layer 51 is made of materialIn_(z)Ga_(1-z)N and 0<z<1. The thickness of the well layer 51 is rangingfrom 3.5 nm to 7 nm, preferably from 4 nm to 5 nm.

The first type electrode 31 and the second type electrode 71 are usedtogether to provide electric power and are made of (but not limited to)the following materials titanium, aluminum, gold, chromium, nickel,platinum, and their alloys. The manufacturing processes are well-knownto people skilled in the art.

Moreover, a first type carrier blocking layer 4 made of materialAl_(x)Ga_(1-x)N (0<x<1) is disposed between the light emitting layer 5and the first type doped semiconductor layer 3 while a second typecarrier blocking layer 6 made of material Al_(x)Ga_(1-x)N (0<x<1) isdisposed between the light emitting layer 5 and the second type dopedsemiconductor layer 7. Due to the property that the band gap of AlGaNcontaining aluminum is larger than that of GaN, not only the range ofthe band gap of the nitride semiconductor is increased, the carriers arealso confined in the MQW structure. Thus the electron-hole recombinationrate is increased and the light emitting efficiency is further improved.

A buffer layer 2 made of Al_(x)Ga_(1-x)N (0<x<1) is disposed between thesubstrate 1 and the first type doped semiconductor layer 3 so as toimprove lattice constant mismatch caused by the first type dopedsemiconductor layer 3 grown on the heterogeneous substrate 1. The bufferlayer 2 can also be made of material including GaN, InGaN, SiC, ZnO,etc.

In summary, due to that both quaternary AlGaInN barrier layers 52 andternary InGaN well layers 51 have the same element-indium, thequaternary composition of the semiconductor light emitting device of thepresent invention can be adjusted and improved for providing a latticematching composition that allows the barrier layers 52 and the welllayers 51 to have similar lattice constants. Thus not only crystaldefects caused by lattice mismatch between conventional InGaN welllayers and GaN barrier layers can be improved, the stress caused bylattice mismatch is also improved. The thickness of the well layer 51 ofthe nitride semiconductor structure is ranging from 5 nm to 7 nm,preferably from 4 nm to 5 nm. Moreover, the addition of more aluminum(Al) in the barrier layer 52 provides a better carrier confinement andelectrons and holes are effectively confined in the well layer 51.Thereby the internal quantum efficiency is increased and thesemiconductor light emitting device provides a better light emittingefficiency.

Furthermore, the quaternary AlGaInN barrier layers and the ternary InGaNwell layers can improve the stress caused by lattice mismatch andfurther reduce the piezoelectric field in the MQW structure effectively.Thus the piezoelectric effect is inhibited and the internal quantumefficiency is improved. Therefore the semiconductor light emittingdevice gets a better light emitting efficiency.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A nitride semiconductor structure comprising: afirst type doped semiconductor layer; a light emitting layer, comprisinga multiple quantum well (MQW) structure, wherein the MQW structurecomprises a plurality of barrier layers and a plurality of well layersstacked alternately; an GaN based hole supply layer comprising indium,doped with a second type dopant at a concentration larger than 10¹⁸ cm⁻³and doped with carbon at a concentration larger than 10¹⁷ cm⁻³; an GaNbased second type carrier blocking layer comprising aluminum, whereinthe GaN based hole supply layer is disposed between the light emittinglayer and the GaN based second type carrier blocking layer; and a secondtype doped semiconductor layer, wherein the GaN second type carrierblocking layer is disposed between the second type doped semiconductorlayer and the light emitting layer, and the light emitting layer isdisposed between the GaN second type carrier blocking layer and thefirst type doped semiconductor layer, wherein the GaN second typecarrier blocking layer is sandwiched between the GaN based hole supplylayer and the second type doped semiconductor layer.
 2. The nitridesemiconductor structure as claimed in claim 1, wherein thicknesses ofthe well layers range from 3.5 nm to 7 nm, and thicknesses of thebarrier layers range from 5 nm to 12 nm.
 3. The nitride semiconductorstructure as claimed in claim 1, wherein each of the barrier layers isdoped with a first type dopant at a concentration ranging from 10¹⁶ cm⁻³to 10¹⁸ cm⁻³.
 4. The nitride semiconductor structure as claimed in claim1, wherein a band gap of the GaN based hole supply layer is larger thana band gap of the well layer of the multiple quantum well structure. 5.The nitride semiconductor structure as claimed in claim 1, wherein thesecond type dopant comprises magnesium or zinc.
 6. A nitridesemiconductor structure comprising: a first type doped semiconductorlayer; a light emitting layer, comprising a multiple quantum well (MQW)structure; an GaN based hole supply layer comprising indium, doped witha second type dopant at a concentration larger than 10¹⁸ cm⁻³ and dopedwith carbon at a concentration larger than 10¹⁷ cm⁻³, wherein the GaNbased hole supply layer directly contacts the MOW structure; a secondtype doped semiconductor layer, wherein the light emitting layer isdisposed between the first type doped semiconductor layer and the GaNbased hole supply layer, and the GaN based hole supply layer is disposedbetween the light emitting layer and the second type doped semiconductorlayer, and the MQW structure comprises a plurality of barrier layers anda plurality of well layers stacked alternately, and the band gap of thehole supply layer is larger than that of the well layers.
 7. The nitridesemiconductor structure as claimed in claim 6, wherein one of thebarrier layers is disposed between the GaN based hole supply layer andone of the well layers.
 8. The nitride semiconductor structure asclaimed in claim 6, wherein thicknesses of the well layers range from3.5 nm to 7 nm, and thicknesses of the barrier layers range from 5 nm to12 nm.
 9. The nitride semiconductor structure as claimed in claim 6,wherein each of the barrier layers is doped with a first type dopant ata concentration ranging from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³.
 10. The nitridesemiconductor structure as claimed in claim 6, wherein the second typedopant comprises magnesium or zinc.
 11. A nitride semiconductorstructure comprising: a first type doped semiconductor layer; an GaNbased first type carrier blocking layer comprising aluminum; a lightemitting layer, comprising a multiple quantum well (MQW) structure,wherein the MQW structure comprises a plurality of barrier layers and aplurality of well layers stacked alternately; an GaN based hole supplylayer comprising indium, doped with a second type dopant at aconcentration larger than 10¹⁸ cm⁻³ and doped with carbon at aconcentration larger than 10¹⁷ cm⁻³; an GaN based second type carrierblocking layer comprising aluminum, wherein the GaN based hole supplylayer is disposed between the light emitting layer and the GaN basedsecond type carrier blocking layer; and a second type dopedsemiconductor layer, wherein the light emitting layer is disposedbetween the first type doped semiconductor layer and the second typedoped semiconductor layer, the GaN based first type carrier blockinglayer is disposed between the first type doped semiconductor layer andthe light emitting layer, and the GaN based second type carrier blockinglayer is disposed between the second type doped semiconductor layer andthe light emitting layer, wherein the GaN based second type carrierblocking layer is sandwiched between the GaN based hole supply layer andthe second type doped semiconductor layer.
 12. The nitride semiconductorstructure as claimed in claim 11, wherein thicknesses of the well layersrange from 3.5 nm to 7 nm, and thicknesses of the barrier layers rangefrom 5 nm to 12 nm.
 13. The nitride semiconductor structure as claimedin claim 11, wherein each of the barrier layers is doped with a firsttype dopant at a concentration ranging from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. 14.The nitride semiconductor structure as claimed in claim 11, wherein aband gap of the GaN based hole supply layer is larger than a band gap ofthe well layer of the multiple quantum well structure.
 15. The nitridesemiconductor structure as claimed in claim 11, wherein the second typedopant comprises magnesium or zinc.
 16. A nitride semiconductorstructure comprising: a first type semiconductor layer; a light emittinglayer comprising a multiple quantum well (MQW) structure, wherein theMQW structure comprises a plurality of GaN based barrier layers and aplurality of GaN based well layers comprising indium, the barrier layersand the well layers are stacked alternately; a second type GaN basedhole supply layer comprising indium, doped with a second type dopant ata concentration larger than 10¹⁸ cm⁻³ and doped with carbon at aconcentration larger than 10¹⁷ cm⁻³, wherein the GaN based hole supplylayer directly contacts the MQW structure; and a second typesemiconductor layer, wherein the light emitting layer is disposedbetween the first type semiconductor layer and the second type GaN basedhole supply layer, and the second type GaN based hole supply layer isdisposed between the light emitting layer and the second typesemiconductor layer.
 17. The nitride semiconductor structure as claimedin claim 16, further comprising a second type GaN based carrier blockinglayer comprising aluminum disposed between the second type GaN basedhole supply layer and the second type semiconductor layer.
 18. Thenitride semiconductor structure as claimed in claim 16 furthercomprising a first type GaN based carrier blocking layer comprisingaluminum disposed between the first type semiconductor layer and thelight emitting layer.
 19. The nitride semiconductor structure as claimedin claim 16, wherein the GaN based barrier layers are doped with a firsttype dopant at a concentration ranging from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³.